The present invention relates to a method of operating a nuclear reactor, and also to a nuclear reactor construction enabling it to be operated in accordance with the novel method. The invention is particularly applicable to heavy-water moderated power reactors, and is therefore described below with respect to such application.
Nuclear reactors moderated by heavy water provide a number of advantages as compared to those using ordinary water as a moderator. One advantage is that it is possible to use natural uranium as well as slightly enriched uranium as the fuel. Another advantage is that the burn-up is high. Such reactors, however, are subject to a number of disadvantages, besides the high price of heavy water. Thus, they exhibit a "positive void coefficient," which means that if heavy water is lost, the reactivity rises thereby increasing the danger of a "run-away." In addition, even though the burn-up rate is high, such reactors still require large amounts of natural uranium, the present supplies of which are extremely limited.